Field of the Invention
The present invention relates to a method of manufacturing a honeycomb structure. More particularly, the present invention relates to a method of manufacturing a honeycomb structure (circumferentially coated honeycomb structure) that includes a circumferential coat layer excellent in surface smoothness with a uniform thickness by applying a slurry-like circumferential coating material on a circumferential surface.
Description of the Related Art
Conventionally, a ceramic honeycomb structure is used for various broad applications such as a catalyst carrier for automobile exhaust-gas purification, a diesel particulate removal filter, a gasoline particulate removal filter, and a heat storage body for a combustion apparatus. Herein, a ceramic honeycomb structure (hereinafter, simply referred to as “honeycomb structure”) is manufactured by performing extrusion molding from an extrusion die by using a honeycomb formed body manufacturing apparatus to obtain a honeycomb formed body and then by further firing the honeycomb formed body at high temperature by using a firing furnace. As a result, it is possible to manufacture a honeycomb structure that includes a porous partition wall defining a plurality of cells of which each forms a fluid through the channel and extends from one end face to another end face.
In recent years, a honeycomb structure used for a catalyst carrier for exhaust gas purification, a particulate removal filter, etc. of an automobile engine requires to improve purification performance in order to cope with exhaust gas regulations that are strengthened year by year from the consideration for environmental problems. In order to correspond to this request, reducing the weight of the honeycomb structure is required with the aim of raising a rate of temperature increase of a catalyst loaded in the honeycomb structure and early activating the catalyst.
For that reason, there has been advanced the development of “wall-thinning” of thinning the thickness of the porous partition wall of the honeycomb structure and “high porosity” of further raising the porosity of the porous partition wall. On the other hand, a demerit accompanied by the wall-thinning and the high porosity of the honeycomb structure may include the decrease in the mechanical strength of the honeycomb structure itself. In other words, for the sake of the wall-thinning, the high porosity, etc., the mechanical strength of the partition wall itself may be decreased compared to the conventional.
As a result, even if a weak impact is externally added to the honeycomb structure, cracks, breakage, etc. may appear on the partition wall. As described above, if cracks, chipping, etc. occur in the partition wall of the honeycomb structure due to an impact, etc., a basic function is impaired when it is used as a catalyst carrier for exhaust gas purification of an automobile, and the likes. For that reason, it is expected to develop a honeycomb structure that achieves the high porosity of the honeycomb structure and has the practical sufficient mechanical strength.
Moreover, a honeycomb structure corresponding to various industrial, technical fields may be manufactured, and thus a large-sized honeycomb structure having a honeycomb diameter larger than the usual is developed. When the large-sized honeycomb structure is integrally formed by extrusion molding, a shape of the partition wall, etc. particularly located at the circumferential periphery is not stabilized, and thus the product shape and dimensional accuracy of the honeycomb structure may be decreased.
Therefore, in order to resolve the above trouble, a circumferential coat layer (circumferential wall) may be provided by grinding the circumferential surface of the extruded honeycomb structure by using a grinding wheel etc. to constantly adjust a honeycomb diameter and then applying a circumferential coating material that contains a powdery ceramic raw material and is prepared in a slurry state on the circumferential surface (grinding surface) of the honeycomb structure to be dried or fired (e.g., see Patent Document 1 and Patent Document 2). As a result, even in the case of the large-sized honeycomb structure having a large honeycomb diameter, a product shape etc. can be stabilized.
As above, the honeycomb structure (circumferentially coated honeycomb structure) on which the circumferential coat layer (circumferential wall) is provided has the improved mechanical strength. Furthermore, because the circumferential surface is previously ground to adjust a honeycomb diameter and then the circumferential coating material is applied to form the circumferential coat layer, the honeycomb structure has an advantage that the improvement (improvement of impact resistance) of the above mechanical strength is performed and it is excellent in dimensional stability of constantly maintaining the size of the final product shape.
A circumferential coat layer forming process for forming the circumferential coat layer is mainly performed by using a coating applicator 100 as schematically shown in FIG. 6, for example. The coating applicator 100 is configured to mainly include: a turntable 103 that places thereon a target honeycomb structure 102 on which a circumferential coat layer 101 is formed and rotates at constant speed in a prescribed rotation direction A by using an axial direction as a rotation center C; and a discharge nozzle 106 that is close to a circumferential surface 104 of the honeycomb structure 102 placed on the turntable 103 and further causes a discharge opening 105 to face the circumferential surface 104.
The discharge nozzle 106 is connected to the other configuration such as a tank (not shown) that retains a circumferential coating material 107 and a discharge mechanism (not shown) that adds a predetermined discharge pressure P to the circumferential coating material 107 to discharge the material from the discharge opening 105. As a result, the circumferential coating material 107 can be discharged from the discharge opening 105 onto the circumferential surface 104 of the honeycomb structure 102.
On the other hand, because the honeycomb structure 102 placed on the turntable 103 rotates at a constant speed in the constant rotation direction A, the relative positional relationship between the circumferential surface 104 of the honeycomb structure 102 and the discharge opening 105 of the discharge nozzle 106 changes from moment to moment. For that reason, the circumferential coating material 107 discharged with a prescribed discharge pressure is applied onto the rotated honeycomb structure 102 in a constant amount each time. Further, drying is performed thereon to form the circumferential coat layer 101 having a uniform thickness, and the honeycomb structure 102 including the circumferential coat layer 101 is manufactured. In addition, FIG. 6 does not show the configuration of cells, a partition wall, etc. on the honeycomb structure 102 for simplification.
[Patent Document 1] JP2613729
[Patent Document 2] JP5345502
The formation of the circumferential coat layer using the above coating applicator may cause the following trouble. In other words, the adjustment of viscosity of the slurry-like circumferential coating material is performed to adapt the circumferential coating material to the coating onto the circumferential surface of the honeycomb structure. In this case, the adjustment of the viscosity is mainly performed by changing the addition amount of water with respect to powdery ceramic raw material acting as the raw material of the circumferential coating material. More specifically, a kneaded object is previously prepared by mixing and kneading a ceramic raw material, water, etc. to have higher viscosity than standard viscosity and then water is gradually added to the kneaded object so as to perform fine adjustment up to viscosity suitable for the coating onto the circumferential surface.
If the viscosity of the circumferential coating material is high, a sufficient amount of circumferential coating material is not applied on the circumferential surface of the honeycomb structure, and thus the thickness of the circumferential coat layer may become thin or uncoating may occur in a portion of the circumferential surface. Additionally, there is a possibility that large unevenness is generated on the surface of the circumferential coat layer. In this case, the thickness of the circumferential coat layer becomes non-uniform, and thus the dimensional stability of the honeycomb structure may be inferior.
Furthermore, the coating amount of the circumferential coating material becomes non-uniform, and thus a crack is easy to occur during drying. Moreover, in the circumferential coated honeycomb structure, bar-code printing, etc. may be generally performed on the surface of the circumferential coating layer with the aim of the product management after manufacturing. In this case, the printed bar-code becomes indistinct due to the ununiformity of the thickness of the circumferential coat layer, and thus reading error by a bar-code scanner may occur frequently.
On the other hand, if the viscosity of the circumferential coating material is low, the circumferential coating material applied on the circumferential surface of the honeycomb structure may drop due to a gravitational force. For that reason, the thickness of the circumferential coat layer may become non-uniform at one end face (upper end side) and another end face (lower end side) of the circumferentially coated honeycomb structure. As a result, the same problem as that of the case where viscosity is high may occur.
Moreover, the supplied amount of the circumferential coating material becomes more as the viscosity is lower, and thus much circumferential coating material is applied on the circumferential surface. In this case, the surplus circumferential coating material is scraped off by a spatula etc. close to the discharge opening. For that reason, the cost for the circumferential coating material may be raised. In other words, it is extremely difficult to adjust the viscosity of the circumferential coating material to maintain the viscosity in an appropriate state, the processing cost accompanied with the decrease in yield and the increase in the used amount of the circumferential coating material is increased, and an excessive burden is imposed on an operator for the sake of the adjustment.
The slurry-like circumferential coating material formed by mixing and kneading ceramic powder and water has a property that the viscosity changes in accordance with a flow velocity (discharge speed). In other words, as a shear rate increases, the viscosity (viscous property) decreases and thus the circumferential coating material has a state in which the coating is easily performed. The present applicant focuses attention on the above characteristics of the slurry-like circumferential coating material, and has found that the trouble as described above can be resolved by appropriately controlling the discharge speed (flow velocity) of the circumferential coating material discharged from the discharge nozzle.
In other words, the circumferential coating material can be controlled so that the material has low viscosity and is excellent in coatability when applying the circumferential coating material on the circumferential surface of the honeycomb structure and the material has high viscosity and does not drip from the circumferential surface at the step at which the coating of the circumferential coating material onto the circumferential surface is completed.
Therefore, the present invention has been achieved in view of the above problems, and an object of the invention is to provide a method of manufacturing a honeycomb structure in which the discharge speed of the circumferential coating material is controlled to be able to form the circumferential coat layer having a uniform thickness.